1. Technical Field
Exemplary embodiments of the present invention relate to a pseudo-satellite transmitter and a method of transmitting GPS signals using a pseudo-satellite transmitter.
2. Discussion of Related Art
A global positioning system (GPS) is a navigation satellite system that provides reliable positioning, navigation, and timing services to worldwide users on a continuous basis. A receiver of a GPS system calculates its position by timing the signals sent by GPS satellites high above the Earth. Each satellite continually transmits messages which may include the time the message was sent, orbital information, general system health, rough orbits of all GPS satellites, etc. The GPS receiver utilizes the messages it receives to determine the transit time of each message and computes the distances to each satellite.
GPS has been used by the U.S. Navy since the early 1970s and was opened to the public in 1984. By the early 2000s, the number of GPS users had rapidly increased and the GPS had become part of vehicle navigation systems and location-based services (LBS). LBS is an information and entertainment service, accessible with mobile devices through the mobile network and utilizing the ability to make use of the geographical position of the mobile device. LBS can include mobile commerce when taking the form of coupons or advertising directed at customers based on their current location, personalized weather services, location-based games etc.
However, GPS is limited to the environment in which the GPS receiver is located. For example, obstructions and interferences in the environment between the GPS receiver and the satellites may prevent the GPS receiver from receiving the signals it needs and at the quality level required to perform calculations. A GPS receiver typically needs to receive four or more GPS satellite signals to determine the coordinates of a three-dimensional position. If one or more of these signals is not received, the GPS receiver may be able compensate for the missing information by utilizing other information that is present.
For example, a GPS receiver may make use of information provided from a pseudo-satellite in a shadow area. The shadow area is an area where three or less signals are received or the quality of the received signal is poor due to a bad reception environment. A base station that transmits GPS signals, such as GPS satellite signals, to the shadow area is called the pseudo-satellite.
The signals transmitted from the GPS satellites are based on a Code division multiple access (CDMA) system where an orthogonal code of each signal is used to discriminate a satellite ID. CDMA signals share a frequency and time. Therefore, when all the received signals do not have the same power level, interference between a signal with a weak power level and a signal with a strong power level occurs, which is called the near-far problem.
Since all of the GPS satellites are disposed at similar heights (e.g., about 25,000 Km from the ground), there is little variation in the intensity of the received signal. Therefore, the near-far problem does not arise. However, when the pseudo-satellite is used, the base station is disposed at a height which is quite different from that of the actual GPS satellites (e.g., several tens of meters to several hundreds of meters). Therefore, there is a large variation in the intensity of the received signal from the pseudo-satellite as compared to the signals from the actual GPS satellites.
Thus, there is a need for a GPS system that is capable of interfacing with a pseudo-satellite that can improve GPS calculations using GPS signals from GPS satellites and pseudo-satellites, and devices and methods for implementing the GPS system.